I closed a question after being satisfied with the answer  (given by a
non google researcher) how can I send a thank-you note to:
 grif-ga 

seems that I cannot do it after I close the question?

Unfortunately us non-researchers cannot be paid for questions. 
Comments can be left after a question is closed so you can leave a
thank you there.  Other options are to request the coomentor leave
his/her email so that you may correspond to them directly or arrange
other contact information although either method can result in others
tampering in your personal business.

Better still, post a Comment to ID 161982.

EVERYBODY who is ANYBODY posts a Comment there.

Also, you (or your look-alike) will get the additional opportunity to
thank the Researcher IN PERSON at the Masked Ball.

There is no charge for this advice!